Air cushion vehicles

ABSTRACT

An air cushion vehicle having a deck and a plurality of flexible inflatable skirts depending downwardly from the deck and then inwardly and upwardly under the deck to enclose air conduit spaces supplied with air from a blower at the rear of the deck. Narrow elongated bottom skirts extend forwardly under the opposite sides of the vehicle where they are respectively secured to the lowermost portions of the inflatable skirts and held in proper vertical disposition by stiffener members secured to the inflatable skirts. The space under the vehicle deck below the reentrant portions of the inflatable skirts forms a rear lifting plenum, while a front lifting plenum is formed below the front of the deck by means including a transverse flexible partition extending under the deck and being partly connected to reentrant portions of the inflatable skirts. The upper edge of the partition terminates just below the deck to define an inlet port through which air passes into the front plenum from air conduits. The lower portion of the partition is slightly spaced above the lower side edges of the vehicle and defines a discharge port through which air exits from the front plenum rearwardly into the rear plenum.

United States Patent [72] Inventor Melville W. Beardsley 40 Windward Drive, Severna Park, Md. 21 146 [2]] Appl. No. 779,576

[22] Filed Nov. 27, 1968 Continuation-impart of application Ser. No. 524,764, Feb. 3, 1966, now Patent No. 3,429,395.

[45] Patented Feb. 16, 1971 [54] AIR CUSHION VEHICLES 22 Claims, 8 Drawing Figs.

[52] US. Cl. 180/118, 180/124, 180/127 [51] Int. Cl B60v 1/04, B60v 1/16 [50] Field ofSearch 180/124, 127,128, 126,118,117

[5 6] References Cited UNITED STATES PATENTS 3,140,687 7/1964 Beardsley 180/126X 3,232,366 2/1966 Cockerell 180/124 3,247,921 4/1966 {.atjrner-Needham et al. 180/124 Primary Examiner-A. Harry Levy Attorney-Beveridge & De Grandi ABSTRACT: An air cushion vehicle having a deck and a plurality of flexible inflatable skirts depending downwardly from the deck and then inwardly and upwardly under the deck to enclose air conduit spaces supplied with air from a blower at the rear of the deck. Narrow elongated bottom skirts extend forwardly under the opposite sides of the vehicle where they are respectively secured to the lowermost portions of the inflatable skirts and held in proper vertical disposition by stiffener members secured to the inflatable skirts. The space under the vehicle deck below the reentrant portions of the inflatable skirts forms a rear lifting plenum, while a front lifting plenum is formed below the front of the deck by means including a transverse flexible partition extending under the deck and being partly connected to reentrant portions of the inflatable skirts. The up er edge of the partition terminates just below the deck to de me an inlet port through which air passes into the front plenum from air conduits. The lower portion of the partition is slightly spaced above the lower side edges of the vehicle and defines a discharge port through which air exits from the front plenum rearwardly into the rear plenum.

"P mdreb. 16,1971 3,563,333

2 Sheets-Sheet 1 MELVlLLE W. BEARDSLEY,

'INVENTOR B My M f Y ATTORNEYS Patented Feb.- .16, 1971 2 Sheets-Sheet 2 MELVILLE W. BEARDSLEY.

ATTORNEYS 1 1 AIR cusnlo vsnrcuas RELATED APPLICATIONS This application is a continuationin-part of my copending U.S. application, Ser. No. 524,764 entitled Air Cushion Vehicle filed Feb. 3, 1966 now US. Pat. No. 3,429,395.

SUMMARY OF INVENTION This invention relates to improvements in air cushion vehicles particularly those such as disclosed in my copending U.S. application identified above.

It is an object of the present inv'entionto improve the pitch A still further object of the present invention is to provide in an air cushion vehicle, novel structure for maintaining a flexible skirt in proper vertical disposition below the vehicle despite the air pressure exerted'on the skirt from within the plenum surrounded thereby.

Yet another object of the present invention is to provide such an air cushion vehicle which will achieve the above ob- FIG. I is a longitudinal cross-sectional view of an air cushion vehicle embodying the invention and with arrows showing the direction of air flow through the vehicle;

FIG. 2 isa bottom plan view of the vehicle shown in FIG. 1 with portions broken away;

FIG. 3 is an elevational view of the vehicle taken from the rear end thereof; a

FIG. 4 is a transverse cross-sectional view taken generally along lines 4-4 of FIG. 2 and with certain portions removed;

FIG. 5 is a transverse cross-sectional view taken generally along lines 5-5 of FIG. 2;

I FIG. 6 is a longitudinal cross-sectional view of an air cushion vehicle representing a second embodiment of the invention;

FIG. 7 is a bottom plan view ofthe rear end portion of the vehicle shown in FIG. 6; and

FIG. 8 is a partly cross-sectional view taken generally along lines 8-8 0F FIG. 7.

. small air cushion vehicle embodying the invention including a horizontal deck 2 an air supply mechanism consisting of a fan 11 driven by a suitable engine 12 mounted to blow air into a duct 13 having a first discharge opening 14 communicating jects while having a minimum weight and being relatively ble skirt portions is a rear lifting plenum which receives its air from a front plenum formed below the deck at the front of the vehicle. The frontplenum receives its air from the air conduits enclosed by the inflatable skirtsthrough a transversely extenda ing inlet port situated just below the deck of thevehicle. Air passes from the front plenum rearwardly into the rear lifting plenum through a discharge port extending transversely just above the plane of the lower edges of the vehicle. These ports are formed by a transversely extending flexible partition the rear side edges of which are connected to opposite reentrant portions of the inflated skirts to merge with them at the center portion of the deck from which the partition curves downwardly in the longitudinal direction of the vehicle and then upwardly and forwardly terminating just below the deck to define the inlet port. The lowermost portion of the partition together with the surface of travel'defines the discharge port.

When the vehicle pitches downwardly towards the traveled surface the discharge port reduces in area thereby increasing the pressure in the front plenum and restoring the vehicle to a stabilized condition. Air discharges from the rear lifting plenum rearwardly of the vehicle below a flexible and inflata: ble rear skirt which curves under and forwardly of the vehicle above the lower side edges of the vehicle.

The lowermost side edges of the vehicle are formed by elongated narrow flexible lower skirts attached to the lower portions of the inflatable'skirt members and maintained in vertical disposition by a plurality of stiffener members secured to the inflatable skirt members and the lower skirts. To provide roll stability, the front plenum chamber may be divided into two transversely spaced compartments-by a longitudinally extending vertical partition secured to the deck generally at the center thereof.

Other objects and advantages will become apparent from the following more detailed description taken in conjunction with the attached drawings in which:

rea'rwardly of the vehicle with atmosphere and a second discharge opening 15 communicating; with a space under the rear portion of the deckas will be described in greater detail.

Depending from the deck are a plurality of flexible inflatable skirt members 3, 4 and 5 formed from thin flexible sheet material such as an elastomer or vinyl coated nylon fabric.

Rear skirt member 3 is attached to the rear transverse edge of deck 2 and curves outwardly and downwardly under the vehicle and then upwardly and forwardly of the vehicle where it is anchored in any suitable manner to the underside of the deck at area 6 which lies on the longitudinal axis of the vehicle. Rear skirt member 3 has converging side edges represented by lines 7, 8 which curve upwardly and forwardly of the vehicle to provide a transverse conduit 16 which may be termed a rear transverse conduit plenum.

Skirt members'4 and 5 depend fromthe opposite side portions of the deck curving outwardly and downwardly under the vehicle and then upwardly to area 6 of the deck where they are anchored together with the rear skirt member 3. The

curvature of side skirt members 4and 5 is illustrated in FIG. 4.

As shown in FIG. 2, the rear edges of side skirt members 4 and 5 are connected to the forwardly extending edges of rear skirt member 3 along lines 7 and 8 and thus in effect provide side conduit plenums 22 extending forwardly on opposite sides of the vehicle from rear transverse'conduit plenum 16. A second transverse conduit plenum termed a forward transverse conduit plenum 23 is formed by an internal partition 27 member formed of similar flexible sheet material having rear converging side edges secured to the forward side edges of side skirt members 4 and 5 along lines 9 and Ill to merge upwardly and be anchored therewith at deck area 6. The front section of partition 27 extends to opposite sides of the vehicle formed by vertically depending front skirt portions 40 best illustrated in FIGS. 4 and 5.

The forward transverse edge or lip 25 of the partition terminates just below the deck and is fixed in position by any suitable means such as a cross rod 42 the opposite ends of which are secured to the opposite side skirts 40.

Forwardly of the partition 27 there is formed below the deck a front lifting plenum 31 by means including side skirts 40 and a front skirt 44 depending from the deck at the bow. As will be described in greater detail, the air is supplied into the front plenum 31 to provide an air cushion from the forward transverse conduit plenum 23 with the air passing through a transversely extending inlet port 26 formed between the deck and the upper forward edge 25 of partition 27.

inasmuch as the side skirt members 4 and are attached to the rear skirt member 3 and partition 27 and merge upwardly to the common area 6 on the deck where they are anchored, the space 30 below the deck area 6 enclosed by'the upwardly converging surfaces of the skirt members and the partition. defines a rear lifting plenum Partition 27 lS dimensioned such that when inflated aswill be described. its lowermost portion 33 will be spaced above the lower side edges of the vehicle to define a transversely extending discharge port 20 communicating the front lifting plenum 31 with the rear lifting plenum 30.

It will bev seen that if the vehicle pitches forwardly and downwardly towards the surface traveled, the area of discharge port 20 will be reduced and the pressure in the front plenum 31 will increase to restore the vehicle to a stable position. A second discharge port 21 is provided at the rear of the vehicle below the lowermost portion of rear skirt member 3 which is raised slightly above the lowermost portion of rear skirt member 3 which is raised slightly above the lowermost side edges of the vehicle as is disclosed in my copending US. Pat. application identified above. Discharge port 21 communicates with the rear lifting plenum 30 to discharge air'rearwardly of the vehicle while being variable in area for stabilization similar to port 20. I

If it is desired to provide roll stability, a longitudinally extending vertical partition 46 may be secured to the deck at the longitudinal center line so as to depend downwardly in front plenum 31 to divide the same into two equal transversely spaced compartments. The lowermost edge 47 of vertical partition 47 terminates at about the same level as the lowermost partition 33 of transverse partition 27. In the event the vehicle should roll to one side, the plenum compartment on that side will increase in pressure to restore the vehicle.

The lowermost edges of the vehicle under the skirt members 4 and are formed by narrow elongated flexible skirt 50 which.may be the same material described above as the skirt members defined above. Lower skirt member 50 is secured to and along the lowermost portions of skirt members 4 and 5. In order to insure that the skirt 50 is maintained in vertical downward position despite the pressure in lift plenums 30 and 31, a plurality of stiffener elements 52 made from suitable rigid plate material secured to the lower portions of skirt members 4 and 5 and also to skirt 50. As shown in FIGS. 4 and 5 the portions of stiffener element 52 secured to skirt 50 ex- .tend generally perpendicular to the portions secured to skirt members 4 and 5. The pressure in the-conduit plenums 16, 22 and 23 will exert a downward component on stiffener elements 52 which will maintain lower skirt 50 downwardly.

In summary it will be seen that the above described skirt structure is formed into its operational shape by the pressure of the air contained within it. The inflating air is supplied by the same blowing means which supplies the air to the supporting air cushion.

The air flow from conduit 13 enters into transverse conduit plenum 16 through opening as indicated by arrow 17. The air then turns laterally outward to travel forward, as indicated by arrow 18, through the side conduit plenums 22 formed by opposite side skirt members 4 and 5 which separate the entire interior plenum from the atmosphere. Entering the forward transverse conduit plenum 23 as indicated by arrow 18, the air flow proceeds through it and exits therefrom, as indicated by arrow 19, through the opening 26 existing between the lower surface of the horizontal deck 2 and the lip of forward interior wall of the flexible enclosure 27.

When the vehicle is in normal level operating position, as indicated with ground surface line 28, the air flow will proceed through port 20 rearwardly under the lowest point 33 of the forward transverse conduit plenum enclosure wall 27 into the rear air cushion plenum 30 existing above the ground surface 28 and under the flexible enclosure walls of the rear transverse conduit plenum 16 the side conduit plenum 22 and the forward transverse conduit plenum 23 respectively. Passing through the rear air cushion plenum 30, the flow escapes through port 21 as indicated by arrow 21ato atmosphere.

The static pressure is substantially the same in the rear transverse conduitplenum 16, the side conduit plenum 22, and the forward transverse conduit plenum 23 since they are interconnected by large opening areas. The air flow from the forward transverse conduit plenum 23 is somewhat restricted as it passes through port 26 into the forward air cushion plenum 31, causing a decrease in static pressure. I

The static pressure in the rear air cushion plenum 30 is slightly lower than in the forward air cushion plenum 31 due to restriction of the flow through port 20. Since the static pressure in the conduit plenums 16, 22, and 23 is greater than the static pressure in the air cushion plenums, 30 and 31, portions of the skirts enclosing the conduit plenums are distended downwardly to form the conduit plenums of shapes determined in accordance with the proportions of the flexible skirt members.

if, during operation, the forward end of the vehicle is displaced downwardly, the flow area of port 20 between the ground surface 28 and the lowest point 33 of transverse partition 27 is reduced, restricting the air flow from the forward air cushion plenum 31 into the rear air cushion plenum 30 and thereby causing an increase of static'pressure in the forward air cushion plenum 31. This increased pressure exerts an increased lifting force on the forward end of the vehicle, causing it to rise until the vehicle regains the normal level position relative to the ground surface 28.

As indicated above, the conduit plenum shapes are determined by the proportions of their flexible enclosure walls which may be comprised of a material such as an elastomercoated fabric. In vertical cross section these conduit plenum walls assume the shape of arcs of a circle, the radius of the circles being determined by the difference in pressure between the inner and outer surfaces.

By the principles of basic mechanics, when a flat flexible membrane, constrained at two parallel ends, is subjected to a pressure difference between its two surfaces it will assume the shape of the arc of a circle between the two constrained parallel ends such that:

2 T= 2 RAp where: v

T lb. per ft. of edge length, exerted in tension at each end and at any section of the membrane parallel to the ends R ft. radius of the are formed A p lb. per -sq. ft. pressure difference between the inner and outer surfaces. To maintain a stable unwrinkled arcuate shape, the tension in the membrane must be uniform.

Since the pressure difference between the interior of the I conduit plenum and atmosphere is greater than the difference in pressure between it and the pressure in the air cushion plenums, the membrane will assume-a different radius for each condition such that:

where:

R, radius of membrane enclosing the conduit plenum within the air cushion plenum R, radius of membrane enclosing the conduit plenum from atmosphere Ap, (pressure in conduit plenum)(pressure in air cushion plenum) Ap, (pressure in conduit plenum)(atmospheric pressure) Thus, 7

. & Z2 'R Ap application, R, is very nearly equal to R so that the'inflated sidewall 72 has a substantially, circular cross section as shown.

In the embodiment described above the entire air flow supply to the supporting air cushion passes through the conduit plenums. This arrangement has been found to be most efficient for small air cushion vehicles; however in the case of larger vehicles it may be desirable to supply a portion, or all, of the air to the air cushion by direct discharge from the blowing means into the front air cushion plenum 31. In the case in which all of the air to the supporting air cushion is supplied directly to the front air cushion plenum the opening 26 would be closed, with the forward edge of the front transverse partition 27 being connected to the lower surface of horizontal deck 2. The air to inflate the conduit plenums could still be supplied by the blowing means supplying the supporting air cushion. This could be accomplished by locating the inlet of the conduit plenums inflation -air supply tube at a point in the blowing means where it would ingest air at maximum total pressure. This maximum total pressure would be greater than the static pressure of the air in the air cushion plenums in that the conduit plenums would be inflated as described.

Referring to FIGS. 6, 7 and 8 there is illustrated another embodiment of the invention which correspondsto the embodiment disclosed in FIGS. 4 and 5 of my copending U.S. Pat. application identified above. In this embodimentthe vehicle includes a horizontal deck 76 a front skirt 77 such as the front skirt 44 in the above-described embodiment. Front skirt 77 depends from the deck 76 and maybe made of suitable sheet material such as an elastomer nylon fabric. By virtue of its shape and attachment to the front and side portions of the deck, skirt 77 will be maintained by the pressure in the plenum 70 in proper position below the deck.

The opposite sides of the vehicle in the presently described embodiment are formed by closed tubular members 72 which extend a substantial distance along the sides of the vehicle as shown in FIG. 6. Tubular members72 are suitably attached to the side edge portions of the deck as shown in FIG. 8 and are prefilled with air and completely closed. The air pressure supplied in tubular members 72 is greater than that which will be developed in the plenum chamber170 so that the tubular members will maintain a circular cross section when the vehicle is in use.

edges of the vehicle below the tubular members are formed by lower elongated skirt portions 78 formed of suitable material as described above. Lower skirt portions 78 are secured along their upper edges to the lowermost portions of tubular members 72. To maintain the lower skirt portions 78 in downward disposition, a plurality of rigid plate stiffeners 80 are attached to tubular members 72 with bottom portions of the stifieners being secured to the lower skirt 78. This arrangement provides a downward and inward force onthe lower skirts 78 to maintain the skirt in proper position. Considering the forces in volved, let arrow 90 represent the outward plenum pressure force on skirt 78 as transmitted to the lower portion of stiffener member 80. This force acting about fulcrum line 91 is balanced by a force represented by arrow 92 acting on stiffener member as a result of the pressure in the tubular member 72. Since the force of stiffener member 80 on skirt 78 is equal and opposite to the force of skirt 78 on the stiffener, by definition, the stiffener 80 therefore exerts an inward force on the skirt. Also, since the pressure force represented 92 on stiffener member 80 has a downward component, a downward force is transmitted to skirt 78 through stiffener 80.

The rear end of the vehicle, is closed by a flexible skirt 94 attached at its upper end to the deck and depending downwardly and under the vehicle and then forwardly and upwardly where it is attached to a cross rod 96 fixed between opposite portions of tubular members 72. The transverse space between the forward edge of the rear skirt member and the deck defines a port 98 through which air entering into the rear plenum 100 moves forwardly into the main plenum 70 which constitutes a lifting pienum. Rear plenum 100 in effect constitutes a conduit plenum. Air is supplied into rear plenum 100 by a suitable blower generally designated 102. As in the As in the above-described embodiment, the lowermost above-described embodiment as well as that disclosed in my copending US. Pat. application identified above, the lowermost portion of rear skirt member 94 is spaced above the lowermost edges of the lower skirts 78 so as to define a discharge opening 104 through which air exits from the main lifting plenum rearwardly of the vehicle.

It will thus be seen that the presently described embodiment incorporates a single main plenum 70 for maintaining the air cushion for lifting the vehicle, and a. conduit plenum l00 which supplies air into the main plenum. Although in the presently described embodiment, tubular members 72 are prefilled with'air, the vehicle including the tubular members may be modified so that the tubular members 72 are inflated by the same air employed to produce the air cushion. In the latter event the modification will be similar to the abovedescribed embodiment.

Iclaim:

1. In an air cushion vehicle includinga generally horizontal deck or the like; the combination comprising means defining a first plenum below the deck for lifting the vehicle including flexible inflatable substantially impenneable first skirt portions on opposite sides of the deck, means connecting said first skirt portions with respect to the deck such that upon inflation sections of said first skirt portions will extend in a direction outwardly of the vehicle in an arcuate shape, second skirt portions of flexible substantially impermeable material joining said first skirt portions adjacent the bottoms of said first skirt portions to extend downwardly from said first skirt portions and longitudinally along the bottoms of said first skirt portions when the latter are inflated, said first and second skirt portions including a plurality of stiffener members located at spaced intervals along said first and second skirt portions to maintain said second skirt portions in generally vertical disposition below said first skirt portions when said first skirt portions are inflated while permitting said second skirt portions to be flexible in the region between the stiffener members, said stiffener members each including a first arm joined to said first skirt portion and a second substantially rigid arm extending at an angle to said first arm and joined to said second skirt portion and extending generally vertically thereof.

2. The air cushion vehicle defined in claim I wherein said first skirt portions are formed by closed tubular members having a continuous generally circular cross section and wherein the pressure in said tubular members is greater than atmospheric pressure to produce an inward force on said stiffener members preventing outward movement of said lower skirt portions.

3. The air cushion vehicle defined in claim 2 wherein said first and second skirt portions are formed from flexible sheetlike material.

4. In the air cushion vehicle defined in claim 1 means defining a second plenum below the deck at the front portion of the vehicle forwardly of said first plenum, air supply means for supplying said second plenum with air, means defining a port between said first and second plenums for discharging air from said second plenum rearwardly into said first plenum for supplying said first plenum with air, said means defining said port including a partition extending transversely between said first skirt portions on the opposite sides of the vehicle generally at the bottom of the vehicle above the surface of travel and located forwardly of the center of the lifting area of the vehicle such that when the deck of the vehicle moves downwardly towards the surface of travel about the center of the lifting area of the vehicle said partition will move closer to the surface of travel to reduce the area of said port and thereby increase the pressure in said second plenum for raising the deck and vehicle to thereby provide longitudinal pitch stability to the vehicle.

5. The air cushion vehicle defined in claim I wherein said first skirt portions extend inwardly under the vehicle with their extremities anchored with respect to the vehicle to define part of said plenum and to enclose a space to be supplied with air to be inflated thereby.

6. in the air cushion vehicle defined in claim wherein the portions of said first skirt portions which extend inwardly under the vehicle have upper and lower surfaces, said plenum being defined in part by the lower surfaces of said first skirt portions and said inflatable space being defined in part by the upper surfaces of said first skirt portions.

7. in the air cushion vehicle defined in claim 6 further including, means defining a second lift plenum at the front portion of the vehicle below the deck, a first port means communicating said inflatable space enclosed by said first skirt portions with said second plenum, second port means communicating said second plenum with said first plenum, and air supply means for supplying air into said inflatable space to flow into said second plenum and then into said first plenum.

8. The air cushion vehicle defined in claim 7 wherein said first and second port means have relatively restricted areas such that the static pressure in said second plenum is less than the pressure in said enclosed space but greater than the static pressure in said first plenum.

9. In the air cushion vehicle defined in claim 8 wherein said second port means is defined by a partition extending transversely under the vehicle adjacent the lower edges thereof and forwardly of the center of lifting area of the vehicle such that when the vehicle moves downwardly about the center of lifting area towards the surface of travel said second discharge port will be reduced in area to increase the pressure in said second plenum for raising the front of the vehicle to provide longitudinal pitch stability.

10 The air cushion vehicle defined in claim 9 wherein said partition extends upwardly towards the deck in the forward direction of the vehicle terminating below the deck to define said first port means.

11. In the air cushion vehicle defined in claim 1 wherein said second skirt portions are shallow in depth relative to said first skirt portions.

12. In the air cushion vehicle defined in claim 11 wherein the bottom edges of said second skirt portions extend linearly into reduce flow of air out of the plenum across the bottom edges for a given clearance height across the second skirts portions over the surface of travel.

13. In an air cushion vehicle having a generally horizontal deck or the like; the combination comprising, means defining a first plenum under the deck for maintaining an air cushion under the deck to lift the vehicle, means defining a second plenum below the deck generally at the front portion of the vehicle for lifting the vehicle, means for introducing air to the second plenum, and means defining a discharge port having a variable area depending on the longitudinally pitched condition ofthe vehicle for discharging air from said second plenum into said first plenum to supply said first plenum with air, said means forming said second plenum including side skirt portions depending downwardly from the deck on opposite sides of the vehicle, an internal transverse partition member extending transversely between opposite sides of the vehicle between said side skirt portions, said partition member having a bottom portion spaced above the lower edges of said side skirt portions to constitute said means defining said discharge port, the distance between said bottom portion of said partition member and the surface of travel being variable when the vehicle pitches longitudinally, said internal partition member having an upper portion extending adjacent to but below said deck to define an inlet port communicating with said second plenum to constitute said means for introducing air into said second plenum.

14. The air cushion vehicle defined in claim 13 further including means for supplying air generally at a rear portion in the vehicle and conduit means extending from said last recited air supply means along the vehicle and communicating with said port for introducing air into said second plenum.

15. The air cushion vehicle defined in claim 14 wherein said conduit means is formed by flexible inflatable skirt members secured to the deck to be inflatable into an arcuate shape.

16. The air cushion vehicle defined in claim 15 wherein said inflatable skirt members define portions of said first plenum.

17. In an air cushion vehicle having a generally horizontal rigid deck with a front end and a rear end, or the like; the combination comprising; means defining a first plenum under the deck for maintaining an air cushion under the deck to lift the vehicle, means defining a second plenum below the deck generally at the front portion of the vehicle for lifting the vehicle, means for introducing air to the second plenum, and means defining a discharge port having a variable area for discharging air from said second plenum into said first plenum to supply said first plenum with air, said means defining said discharge port including a partitionmember extending continuously transversely of the vehicle to opposite sides of the vehicle generally at but above the bottom of the vehicle and the surface of travel and located forwardly of the center of the lifting area of the vehicle such that when the front end of said deck moves downwardly towards the surface of travel about the center of the lifting area of the vehicle said partition member will move closer to the surface of travel to reduce the area of saidport and thereby increase the pressure in said second plenum for raising the deck and vehicle to thereby provide longitudinal pitch stability to the vehicle.

18. The air cushion vehicle defined in claim 15 further including a longitudinally extending partition depending from the deck in the second plenum generally at the longitudinal center line of the vehicle to provide roll stability for the vehicle.

19. in an air cushion vehicle having a generally horizontal deck or the like; the combination comprising means defining air conduits extending partly along opposite sides of the vehicle and a plenum chamber communicating with said air conduits to be supplied with air for lifting the vehicle, said means including opposite sideskirt portions depending from opposite sides of the deck downwardly from the deck and-under the deck, said side skirt portions having upper surfaces defining in part said air conduits and lower surfaces defining in part said plenum, said means further including a partition member extending transversely under the deck between the opposite side skirt portions and having a lower surface partly defining said plenum, said partition terminating at a forward end below said deck to define therewith a port for communicating said conduits with said plenum.

20. An air cushion vehicle comprising in combination, a generally horizontally extending deck, inflatable skirt members depending from the deck and under the vehicle to include inflatable spaces while at the same time to define a plenum chamber below the surfaces of said skirt members for lifting the vehicle, air supply means for supplying air into said inflatable spaces to inflate said skirt members, and duct means communicating said inflatable spaces with said plenum to supply air into said plenum for lifting the vehicle, said skirt members including a rear skirt member attached to the rear end of the deck and extending outwardly and under the vehicle and upwardly and forwardly in curved configuration, side skirt members attached to opposite side portions of the deck and extending outwardly and under the vehicle then upwardly and being secured to opposite side portions of said rear skirt member, said air supply means communicating with the inflatable space enclosed by said rear skirt member, and said rear skirt member converging with said side skirt members toward the deck of the vehicle such that air entering into the space enclosed by the rear skirt member moves transversely at the rear of the vehicle andthen forwardly along the sides of the vehicle.

21. In an air cushion vehicle including a generally horizontal deck or the like; the combination comprising means defining a first plenum below the deck for lifting the vehicle including flexible inflatable first skirt portions on opposite sides of the deck, means connecting said first skirt portions with respect to the deck such that upon inflation sections of said first skirt portions will extend in a direction outwardly of the vehicle in an arcuate shape, second skirt portions of flexible material joining said first skirt portions adjacent the bottoms of said first skirt portions to expend downwardly from said first skirt portions and longitudinally along the bottoms of said first skirt portions including a plurality of stiffener members located at spaced intervals along said first and second skirt portions to maintain said second skirt portions in generally vertical disposition below said first skirt portions when said first skirt portions are inflated while permitting said second skirt portions to be flexible in the region between the stiffener members, said stiffener members each including a first arm joined to said first skirt portion and arsecond arm extending at an angle to said first arm and joined to said second skirt portion and extending generally vertically thereof, said second skirt portions being shallow in depth relative to said first skirt portions, and said second skirt portions being made from relatively thin sheet material having a narrow bottom edge and the surface of travel.

22. in an air cushion vehicle including a generally horizontal deck or the like; the combination comprising means defining a first plenum below the deck for lifting the vehicle including flexible inflatable first skirt portions on opposite sides of the deck, means connecting said firstfskirt portions with respect to the deck such that upon inflation sections of said first skirt portions will extend in a direction outwardly of the vehicle in an arcuate shape, second skirt portions of flexible material joining said first skirt portions adjacent the bottoms of said first skirt portions to extend downwardly from said first skirt portions and longitudinally along the bottoms of said first skirt portions when the latter are inflated, said first and second skirt portions including a plurality of stiffener members located at spaced intervals along said first and second skirt portions to maintain said second skirt portions in generally vertical disposition below said first skirt portions when said first skirt portions are inflated while permitting said second skirt portions to be flexible in the region between the stiffener members said stiffener members each including a first arm joined to said first skirt portion and asecond arm extending at an angle to said first arm and joined to said second skirt portion and extending generally vertically thereof, said second skirt portions being shallow in depthrelative to said first skirt portions, said first skirt portions containing sufficient air pressure 

1. In an air cushion vehicle including a generally horizontal deck or the like; the combination comprising means defining a first plenum below the deck for lifting the vehicle including flexible inflatable substantially impermeable first skirt portions on opposite sides of the deck, means connecting said first skirt portions with respect to the deck such that upon inflation sections of said first skirt portions will extend in a direction outwardly of the vehicle in an arcuate shape, second skirt portions of flexible substantially impermeable material joining said first skirt portions adjacent the bottoms of said first skirt portions to extend downwardly from said first skirt portions and longitudinally along the bottoms of said first skirt portions when the latter are inflated, said first and second skirt portions including a plurality of stiffener members located at spaced intervals along said first and second skirt portions to maintain said second skirt portions in generally vertical disposition below said first skirt portions when said first skirt portions are inflated while permitting said second skirt portions to be flexible in the region between the stiffener members, said stiffener members each including a first arm joined to said first skirt portion and a second substantially rigid arm extending at an angle to said first arm and joined to said second skirt portion and extending generally vertically thereof.
 2. The air cushion vehicle defined in claim 1 wherein said first skirt portions are formed by closed tubular members having a continuous generally circular cross section and wherein the pressure in said tubular members is greater than atmospheric pressure to produce an inward force on said stiffener members preventing outward movement of said lower skirt portions.
 3. The air cushion vehicle defined in claim 2 wherein said first and second skirt portions are formed from flexible sheetlike material.
 4. In the air cushion vehicle defined in claim 1 means defining a second plenum below the deck at the front portion of the vehicle forwardly of said first plenum, air supply means for supplying said second plenum with air, means defining a port between said first and second plenums for discharging air from said second plenum rearwardly into said first plenum for supplying said first plenum with air, said means defining said port including a partition extending transversely between said first skirt portions on the opposite sides of the vehicle generally at the bottom of the vehicle above the surface of travel and located forwardly of the center of the lifting area of the vehicle such that when the deck of the vehicle moves downwardly towards the surface of travel about the center of the lifting area of the vehicle said partition will move closer to the surface of travel to reduce the area of said port and thereby increase the pressure in said second plenum for raising the deck and vehicle to thereby provide longitudinal pitch stability to the vehicle.
 5. The air cushion vehicle defined in claim 1 wherein said first skirt portions extend inwardly under the vehicle with their extremities anchored with respect to the vehicle to define part of said plenum and to enclose a space to be supplied with air to be inflated thereby.
 6. In the air cushion vehicle defined in claim 5 wherein the portions of said first skirt portions which extend inwardly under the vehicle have upper and lower surfaces, said plenum being defined in part by the lower surfaces of said first skirt portions and said inflatable space being defined in part by the upper surfaces of said first skirt portions.
 7. In the air cushion vehicle defined in claim 6 further including, means defining a second lift plenum at the front portion of the vehicle below the deck, a first port means communicating said inflatable space enclosed by said first skirt portions with said second plenum, second port means communicating said second plenum with said first plenum, and air supply means for supplying air into said inflatable space to flow into said second plenum and then into said first plenum.
 8. The air cushion vehicle defined in claim 7 wherein said first and second port means have relatively restricted areas such that the static pressure in said second plenum is less than the pressure in said enclosed space but greater than the static pressure in said first plenum.
 9. In the air cushion vehicle defined in claim 8 wherein said second port means is defined by a partition extending transversely under the vehicle adjacent the lower edges thereof and forwardly of the center of lifting area of the vehicle such that when the vehicle moves downwardly about the center of lifting area towards the surface of travel said second discharge port will be reduced in area To increase the pressure in said second plenum for raising the front of the vehicle to provide longitudinal pitch stability. 10 The air cushion vehicle defined in claim 9 wherein said partition extends upwardly towards the deck in the forward direction of the vehicle terminating below the deck to define said first port means.
 11. In the air cushion vehicle defined in claim 1 wherein said second skirt portions are shallow in depth relative to said first skirt portions.
 12. In the air cushion vehicle defined in claim 11 wherein the bottom edges of said second skirt portions extend linearly into reduce flow of air out of the plenum across the bottom edges for a given clearance height across the second skirts portions over the surface of travel.
 13. In an air cushion vehicle having a generally horizontal deck or the like; the combination comprising, means defining a first plenum under the deck for maintaining an air cushion under the deck to lift the vehicle, means defining a second plenum below the deck generally at the front portion of the vehicle for lifting the vehicle, means for introducing air to the second plenum, and means defining a discharge port having a variable area depending on the longitudinally pitched condition of the vehicle for discharging air from said second plenum into said first plenum to supply said first plenum with air, said means forming said second plenum including side skirt portions depending downwardly from the deck on opposite sides of the vehicle, an internal transverse partition member extending transversely between opposite sides of the vehicle between said side skirt portions, said partition member having a bottom portion spaced above the lower edges of said side skirt portions to constitute said means defining said discharge port, the distance between said bottom portion of said partition member and the surface of travel being variable when the vehicle pitches longitudinally, said internal partition member having an upper portion extending adjacent to but below said deck to define an inlet port communicating with said second plenum to constitute said means for introducing air into said second plenum.
 14. The air cushion vehicle defined in claim 13 further including means for supplying air generally at a rear portion in the vehicle and conduit means extending from said last recited air supply means along the vehicle and communicating with said port for introducing air into said second plenum.
 15. The air cushion vehicle defined in claim 14 wherein said conduit means is formed by flexible inflatable skirt members secured to the deck to be inflatable into an arcuate shape.
 16. The air cushion vehicle defined in claim 15 wherein said inflatable skirt members define portions of said first plenum.
 17. In an air cushion vehicle having a generally horizontal rigid deck with a front end and a rear end, or the like; the combination comprising; means defining a first plenum under the deck for maintaining an air cushion under the deck to lift the vehicle, means defining a second plenum below the deck generally at the front portion of the vehicle for lifting the vehicle, means for introducing air to the second plenum, and means defining a discharge port having a variable area for discharging air from said second plenum into said first plenum to supply said first plenum with air, said means defining said discharge port including a partition member extending continuously transversely of the vehicle to opposite sides of the vehicle generally at but above the bottom of the vehicle and the surface of travel and located forwardly of the center of the lifting area of the vehicle such that when the front end of said deck moves downwardly towards the surface of travel about the center of the lifting area of the vehicle said partition member will move closer to the surface of travel to reduce the area of said port and thereby increase the pressure in said second plenum for raising the deck and vehicle to thereby provide longitudinal pItch stability to the vehicle.
 18. The air cushion vehicle defined in claim 15 further including a longitudinally extending partition depending from the deck in the second plenum generally at the longitudinal center line of the vehicle to provide roll stability for the vehicle.
 19. In an air cushion vehicle having a generally horizontal deck or the like; the combination comprising means defining air conduits extending partly along opposite sides of the vehicle and a plenum chamber communicating with said air conduits to be supplied with air for lifting the vehicle, said means including opposite side skirt portions depending from opposite sides of the deck downwardly from the deck and under the deck, said side skirt portions having upper surfaces defining in part said air conduits and lower surfaces defining in part said plenum, said means further including a partition member extending transversely under the deck between the opposite side skirt portions and having a lower surface partly defining said plenum, said partition terminating at a forward end below said deck to define therewith a port for communicating said conduits with said plenum.
 20. An air cushion vehicle comprising in combination, a generally horizontally extending deck, inflatable skirt members depending from the deck and under the vehicle to include inflatable spaces while at the same time to define a plenum chamber below the surfaces of said skirt members for lifting the vehicle, air supply means for supplying air into said inflatable spaces to inflate said skirt members, and duct means communicating said inflatable spaces with said plenum to supply air into said plenum for lifting the vehicle, said skirt members including a rear skirt member attached to the rear end of the deck and extending outwardly and under the vehicle and upwardly and forwardly in curved configuration, side skirt members attached to opposite side portions of the deck and extending outwardly and under the vehicle then upwardly and being secured to opposite side portions of said rear skirt member, said air supply means communicating with the inflatable space enclosed by said rear skirt member, and said rear skirt member converging with said side skirt members toward the deck of the vehicle such that air entering into the space enclosed by the rear skirt member moves transversely at the rear of the vehicle and then forwardly along the sides of the vehicle.
 21. In an air cushion vehicle including a generally horizontal deck or the like; the combination comprising means defining a first plenum below the deck for lifting the vehicle including flexible inflatable first skirt portions on opposite sides of the deck, means connecting said first skirt portions with respect to the deck such that upon inflation sections of said first skirt portions will extend in a direction outwardly of the vehicle in an arcuate shape, second skirt portions of flexible material joining said first skirt portions adjacent the bottoms of said first skirt portions to expend downwardly from said first skirt portions and longitudinally along the bottoms of said first skirt portions when the latter are inflated, said first and second skirt portions including a plurality of stiffener members located at spaced intervals along said first and second skirt portions to maintain said second skirt portions in generally vertical disposition below said first skirt portions when said first skirt portions are inflated while permitting said second skirt portions to be flexible in the region between the stiffener members, said stiffener members each including a first arm joined to said first skirt portion and a second arm extending at an angle to said first arm and joined to said second skirt portion and extending generally vertically thereof, said second skirt portions being shallow in depth relative to said first skirt portions, and said second skirt portions being made from relatively thin sheet material having a narrow bottom edge and the surface of travel.
 22. In an air cushion vehicle including a generally horizontal deck or the like; the combination comprising means defining a first plenum below the deck for lifting the vehicle including flexible inflatable first skirt portions on opposite sides of the deck, means connecting said first skirt portions with respect to the deck such that upon inflation sections of said first skirt portions will extend in a direction outwardly of the vehicle in an arcuate shape, second skirt portions of flexible material joining said first skirt portions adjacent the bottoms of said first skirt portions to extend downwardly from said first skirt portions and longitudinally along the bottoms of said first skirt portions when the latter are inflated, said first and second skirt portions including a plurality of stiffener members located at spaced intervals along said first and second skirt portions to maintain said second skirt portions in generally vertical disposition below said first skirt portions when said first skirt portions are inflated while permitting said second skirt portions to be flexible in the region between the stiffener members said stiffener members each including a first arm joined to said first skirt portion and a second arm extending at an angle to said first arm and joined to said second skirt portion and extending generally vertically thereof, said second skirt portions being shallow in depth relative to said first skirt portions, said first skirt portions containing sufficient air pressure producing a moment on said first stiffener arm about a fulcrum located at the common juncture of said stiffener arms which moment maintains the second skirt portions downwardly in desired position. 